Chronic heart failure (HF) occurs when a heart is unable to consistently pump blood at an adequate rate in response to the filling pressure. To improve the ability of the heart to pump blood, congestive heart failure patients, classified as having New York Heart Association (NYHA) class status of II to IV HF, may require implantable medical devices (IMDs) such as implantable cardioverter defibrillators (ICDs) and cardiac resynchronization therapy devices with defibrillation capability (CRT-Ds). Despite using IMDs to improve heart function, some HF patients may require hospitalization. Global health care systems incur billions of dollars each year due to heart failure hospitalizations (HFHs). Identifying patients at risk of a heart failure event (HFE) (e.g. HFH) to enable timely intervention and prevent expensive hospitalization remains a challenge. ICDs and CRT-Ds are configured to acquire data for a variety of diagnostic metrics that change with HF status and collectively have the potential to signal an increasing risk of HFE. Diagnostic parameter data collected by IMDs include activity, day and night heart rate, atrial tachycardia/atrial fibrillation (AT/AF) burden, mean rate during AT/AF, percent CRT pacing, number of shocks, and intrathoracic impedance. Additionally, preset or programmable thresholds for diagnostic metrics, when crossed, can trigger a notification, referred to as device observation. Each device observation is recorded in an IMD report and can be transmitted to an external healthcare system. Numerous healthcare systems (i.e. CARELINK® from Medtronic) are able to automatically notify health care workers of potential issues associated with a patient.
While numerous healthcare systems are able to automatically notify healthcare workers of potential health care issues such as that which is disclosed in US Patent Application US 2010-0030293 A1 to Sarkar et al., a health care system typically requires a physician's input to adjust therapy delivered to a patient. It is therefore desirable to store and transmit, in real-time, data to a system that is able to determine an increased heart failure (HF) event risk and is able to seamlessly respond to increased HF event risk.
In addition, impedance readings can include defined ranges such as hysteresis ranges as shown by U.S. Pat. No. 8,055,335 to Stylos, U.S. Pat. No. 8,700,143 to Stylos, but neither of these references determine whether a patient is dehydrated. It is also desirable to be able to predict in real-time risk of hypervolemia or hypovolemia in a patient. Hypervolemia occurs when too much fluid is present in the blood whereas hypovolemia (i.e. dehydration) occurs when too little fluid is present in the blood.